The invention relates generally to voltaic piles and relates more particularly to voltaic piles which are charged and discharged through two terminals and which contain a charge redistribution system for equalizing charge among cells thereof, which charge redistribution system operates at a much lower power level than the full charging or discharging power level of the system.
Much attention has been given in recent years to the efficiency and reliability of rechargeable batteries (voltaic piles). It is desired to have the benefit of a large number of charge/discharge cycles, thus saving the inconvenience, downtime and expense of replacement of the battery and the environmental costs of disposal. But even with the most stringent manufacturing controls it is impossible to manufacture the cells of the pile so that each one has exactly the same capacity as the next, throughout the useful life of the cell. It invariably happens that one cell ages differently than its neighbors. The performance of the pile is essentially limited to the performance of the weakest cell in the pile. When that weakest cells is discharged, the pile becomes unusable.
It is, of course, well-known to perform the recharging of individual cells in a series configuration by means of isolation transformers, with a secondary winding for each cell, shown for example in U.S. Pat. No. 5,412,305 to Jeanneret (issued May 2, 1995). But such an approach does not necessarily take full advantage of the recharging opportunity provided by, for example, regenerative braking in an electric vehicle. And such an approach does not necessarily permit balancing of cell charge in real time during quiescent intervals. In particular it may be bulky and inefficient if used for overall charging of the pile and if also used for low-power charge redistribution.
It is thus desirable to provide a system in which energy is removed from the entire voltaic pile and is used to replace energy into individual cells or groups of cells having the smallest capacity, at a low power level relative to the charge and discharge power levels. It is also desirable to provide a system which takes full advantage of regenerative braking in an electric vehicle.
A voltaic pile has first and second terminals by which it is charged and discharged. The pile comprises a plurality of modules, each having third and fourth respective terminals with a plurality of voltaic cells connected series-wise therebetween. The modules are connected series-wise (or in series-parallel configuration) by the respective third and fourth terminals, with end terminals of the series-wise-connected modules defining the previously mentioned first and second terminals. A power supply is provided with DC input terminals and AC output terminals, the DC input terminals connected with the first and second terminals. Thus, even in the absence of charging current or drain current, the power supply is able to draw upon the output of the pile to supply power back to individual modules that may require charging to maintain balance among the modules. Each of the modules has respective charging terminals connected with the power supply AC output terminals, and further has a transformer with a primary winding connected with the charging terminals and a secondary winding coupled via a rectifier with the cells of the modules, whereby the cells are charged. Importantly, the charge redistribution power level is small compared with the main drain and charge power levels.